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The discovery of the electron: I
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1997 Eur. J. Phys. 18 133
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Eur. J. Phys. 17 (1996) 133138. Printed in the UK 133
The discovery of the electron: I
Nadia RobottiDipartimento di Fisica, Universita` di Genova, via Dodecaneso 33, I-16145 Genova, Italy
Received 2 October 1996
Abstract. This paper describes the process by which the firststudies on discharges in rarefied gases led to the discovery ofthe electron in 1897. Particular emphasis is laid on the debatebetween the so-called aetherial and material theoriesregarding the nature of cathode rays. The paper goes on todemonstrate how the debate was resolved by J J Thomsonwith his proposal of a third hypothesisthe corpuscle (orelectron as it became called). The paper closes with ananalysis of the first measurement of the charge of the electronby J J Thomson in 1899.
Resume. Ce travail etablit comment, a` partir des premie`resetudes sur la decharge dans les gaz rarefies, on put arriver, en1897, a` la decouverte de lelectron. On analyse en particulierle debat qui eut lieu entre lhypothe`se dite etheree etlhypothe`se materialiste quant a` la nature des rayonscathodiques. Ensuite on explique comment ce debat fut clospar J J Thomson, qui proposa une troisie`me hypothe`secelledu corpuscule (ou electron, comme il sera ensuitedenomme). Larticle se termine avec un analyse de lapremie`re mesure de la charge de lelectron realisee parJ J Thomson en 1899.
In 1873 Maxwell made this comment on dischargeprocesses in rarefied gases:
These and many other phenomena: : : are exceed-ingly important, and, when they are better understood,they will probably throw great light on the nature ofelectricity as well as on the nature of gases .
As we will see, history proved Maxwell right. Justa few decades later research of this type led to thediscovery of the electron
1. The first steps
One of the most important challenges faced by physicsin the second half of the 19th century was to understandwhat J J Thomson referred to as the secret ofelectricityor the nature of electricity itself and therelationship between electricity and matter. One pathchosen to do this was to study the behaviour of ararefied gas in the presence of an electrical discharge.The reasons behind the choice were twofold. In thefirst instance this type of study would have led toan analysis of the interactions between electricity andmatter in its evidently simplest state. In the secondplace, for the gaseous state, unlike the other states, therewas a fairly consolidated theory (the kinetic theory)that could be used as a point of reference. In anycase, research of this type could only proceed when(thanks above all to Geissler) vacuum pumps able toproduce pressures of the order of 102103 mm Hgbecame available. At these pressures it was observedthat, while the various areas in the gas behaved indifferent ways depending on the nature of the gas, the
type of electrodes, the shape of the tube, etc, there wasalways a dark space near the cathode, independent ofthe operating conditions. Figure 1 below, taken fromPhilosophical Transactions of 1880 , illustrates anumber of discharge phenomena for pressures around102 mm Hg.
This dark space near the cathode (which had alreadybeen pointed out by Faraday in 1838 ) increased onlyin size as the degree of vacuum increased. At pressuresin the order of 103 mm Hg, the space extended beyondthe anode along the entire length of the tube, becomingthe only phenomenon present.
Once the dark space had been identified as a constantin discharge processes, it became the focus of attentionin an attempt to identify its physical properties. From1860 onwards, thanks above all to the work of Plucker,Goldstein, Varley and Crookes, it was established thatthis dark space was the transit area for somethingderiving from the cathode, which invisible until itmet an obstacle, after which it became evident andcould be perceived. This something, independentlyof the material used for the cathode, had the followingproperties:
(i) it caused phosphorescence in the glass or on anyphosphorescent object placed in its trajectory;
(ii) it was emitted perpendicularly to the cathode andtravelled in a straight line, independently of the positionof the anode;
(iii) it produced chemical reactions, exerted amechanical effect, was deflected by a magnetic field,and created a shadow from any object placed in its path.
So far as concerns the nature of this something,from 1870 two opposing theories were developed.On the one hand, there was the aetherial theory
0143-0807/96/030133+06$19.50 c 1996 IOP Publishing Ltd & The European Physical Society
134 N Robotti
Figure 1. Electric discharge phenomena in different operating conditions.
supported by most physicists of the German schoolstudying the question. Under the terms of this theory,the phenomenon was regarded as an electromagneticprocess, i.e. a wave of small wavelength (this is thereason for the name cathode rays). On the otherhand, there was the material theory supported bymost physicists of the British school, who, using ananalogy with the electrolytic processes studied for sometime, considered the cathode rays to be negativelyionized atoms or molecules (the sign of the chargewas established by the curvature in a magnetic field).Clearly these two theories succeeded in explainingin part the properties of cathode rays discoveredup to that point. It is no coincidence that therewas an immediate conflict between the two theories,above all in experimental terms. Every experimentdesigned and implemented to highlight new propertiesof cathode rays was set up to be able to distinguishbetween these two theories. Despite the objectiveof the various experiments performed, no experiment,taken individually, was sufficient to resolve the debatebetween the two theories, and in fact the debatecontinued at an experimental level until 1897 .
Without entering into details, I shall indicate the mostdifficult phase of the conflict. In 1887 Hertz devised aseries of experiments in order to test the basic hypothesisof the material theory, according to which the cathoderays were electrical in nature . In particular, heverified whether or not they carry a charge and weredeflected by an electric field.
Figure 2 illustrates the apparatus used by Hertz todetect an eventual charge. The vacuum tube was placedinside two coaxial cylinders, and , both connected tothe electrometer. The inner cylinder acted as a chargedetector and the external tube as a screen. Accordingto Hertz, if the cathode rays carried a charge itshould have been communicated by induction across theinner cylinder to the electrometer and detected by it.However, contrary to every prediction of the materialtheory, no charge was signalled.
To verify the eventual action of an electric field,Hertz used an apparatus (similar to the one shown infigure 4) in which the cathode rays were made topass through two metal plates connected to the polesof a battery. If the cathode rays were sensitive to theelectric field, they should have been deflected. Contrary
The discovery of the electron: I 135
Figure 2. Hertzs apparatus to detect an eventualcharge of the cathode rays.
to this prediction of the material theory, there was noperceivable shift of the rays.
Hertzs experiments did not, however, end the debatebetween material and aetherial theories. Quite tothe contrary, the debate not only continued, but nowfocused principally on the problems touched on byHertzs experiments, and eventually led to diametricallyopposed results. Supporters of the material theorywere encouraged by the deflection of the rays in amagnetic field, and at the same time they were able toaccount for Hertzs results in terms of their own theoryby saying that an unexpected effect or an as yet unknownphenomenon had in some way masked the electrostaticcharacteristics of the cathode rays. It is from this pointof view and with the precise purpose of eliminatingspurious effects present in Hertzs experiments, that inmy opinion we must interpret, for example, Perrinsexperiment of 1895 .
Figure 3. Perrins apparatus to detect an eventual charge of the cathode rays.
In 1895 Perrin repeated Hertzs experiment on chargecarrying, but with an important variation. The twocoaxial cylinders ( and ), which in the arrangementadopted by Hertz were placed outside the vacuum tube,in this case were placed inside the tube to eliminate thescreening effect of the glass. The apparatus used byPerrin is illustrated in schematic form in figure 3.
The two metal cylinders ABCD and EFGH had twosmall openings, and , to allow the cathode rays toenter them. The cathode was formed of an electrodeN, and the anode of the protection cylinder EFGH.With this apparatus Perrin observed that when the beamof cathode rays entered cylinder ABCD, invariablythe cylinder became charged with negative electricity.If, however, the equipment was placed in a magneticfield, so that the cathode rays could no longer entercylinder ABCD, the cylinder was not charged. Perrinhence concluded: Cathode rays are therefore chargesof negative electricity.
Despite Perrins results, the material theory stillhad to come to terms with Hertzs objection, when hedemonstrated that the cathode rays were not deflectedby an electric field. Several supporters of the materialtheory had repeated Hertzs experiment but had alwaysobtained the old result. In the meantime, yet anotherobjection to the material theory had arisen.
In 1894 Lenard, one of the most strenuous supportersof the wave nature of cathode rays, used an observationmade by Hertz in 1892 according to whichcathode rays seemed to be capable of passing throughthin metal filmsto design a new type of vacuumtube, in which the wall of the tube opposite the cathode(which in normal conditions blocked the cathode rays)was replaced with a metal film, having small enoughthickness to be passed through by cathode rays (e.g.a thickness of about 0.003 mm for aluminium). Thisisolated the cathode rays from the discharge tube,enabling them to be studied under a wide range ofconditions. Lenard  proceeded with a systematicstudy of the absorption of these rays by the variousmaterials. As cathode rays could pass through metalfilms that were impenetrable to atoms (they were oftenused to separate hydrogen or other gases, on one side,from a good vacuum, on the other), according to Lenardthey could not be considered atoms but had to beregarded rather as waves. This was the state of thedebate on cathode rays when Roentgen announced hisdiscovery of x-rays in 1895 .
136 N Robotti
Figure 4. Thomsons apparatus to detect the deflection of the cathode rays by an electric field.
2. The corpuscle
At the beginning of 1896, when Roentgens firstpapers began to circulate among English physicists, J JThomson (then director of the Cavendish Laboratory inCambridge) was studying the conduction processes ofelectricity in gases. The first thing he did on obtaininga copy of the x-ray apparatus devised by Roentgenwas to verify the effect of these rays on a gas. Herealized that the x-rays ionized the gas, turning it intoa good conductor of electricity. At this point the waywas open to the discovery of the electron. Thomsonchecked whether cathode rays had the same propertiesas x-rays. If this were the case, it would have beenpossible to consider the lack of deflection of the raysin the presence of an electric fieldobserved duringexperimentsas due to the ionization of the gas, whichin some way masked the electric field present. In otherwords, the deflection of the rays was zero becausethe electric field strength was reduced to zero. Bymeans of a series of experiments aimed at establishingthe eventual link between the conductivity conferred onthe gas by the cathode rays and the gas pressure,Thomson arrived at the conclusion that there was aconductivity of the gas, which disappeared very rapidlyas the exhaustion was increased . At this pointThomson repeated Hertzs experiment of 1887, butat reduced pressure (see figure 4), and he observedthe deflection of the rays, even when the potentialdifference (created between D and E) was as small as2 V. Thomson commented : It was only whenthe vacuum was a good one that the deflection tookplace. After verifying that the cathode rays werecarrying a charge and were deflected by a magnetic fieldas well as by an electric field, Thomson concluded asthe only possibility that they are charges of negativeelectricity carried by particles of matter. Then he askedthe new question: What are these particles? Are theyatoms, or molecules, or matter in an still finer stateof subdivision? To answer this question, Thomsondetermined the mass/charge ratio of these particles byusing two different experimental methods.
The first method exploited the deflection of raysboth in an electric and in a magnetic field. The apparatusused is shown in figure 4, where a magnetic field B(of fixed strength) was applied perpendicularly to the
path of the rays, while a variable electric field E wascreated between the plates D and E. Thomson varied Eto that value E0 at which the cathode rays returnedto the undeflected position. Under these conditions thevelocity v of the rays took on the value
v D E0=B: (1)Then the electric field was removed and the radius ofcurvature R of the rays observed and calculated. Onapplying equation (1), Thomson found an expression form=e that contained only observable quantities, namely
m=e D RB2=E0: (2)In the second method, Thomson exploited only the
deflection in a magnetic field. The velocity v of therays was determined, assuming that all the kineticenergy can be transformed into heat, from making useof the formula:
m=e D RB2Q=2W (3)where Q represented the charge passing through asection of the beam in the time unit and W was thekinetic energy associated with it. These quantities weremeasured by Thomson using three different tubes, allof the type devised by Perrin (see figure 3). Thequantity W was measured using a thermocouple ofa known thermal capacity placed behind the centralopening.
These two methods enabled Thomson to obtain avalue for the ratio m=e of the order of 107 g/emu.This value proved out to be independent of the materialused for the cathode, the gas employed, and the pressureapplied. He emphasized, in particular, that it was verysmall compared with the value 104, which was thesmallest value known so far for the mass/charge ratio ofan ion, the hydrogen ion. Per se, this value was rela-tively insignificant unless it was separated into mass andcharge, separately. Thomson, however, succeeded indeducing information from this ratio thanks to Lenardsmeasurement of 1894 of the absorption of cathode raysin air; at a pressure of 0.5 atm, he obtained the meanfree path of the cathode rays and compared it to themean free path of a molecule of air under the same con-ditions. In the case of cathode rays the value came outto be about 0.5 cm, while for air the value was about2105 cm, that is a quantity of a quite different order.
The discovery of the electron: I 137
According to Thomson, cathode rays were new parti-cles of a much smaller mass than ordinary molecules.
Given a constant mass/charge ratio, independently ofthe materials used, it became impossible for Thomsonto avoid not only the conclusion that the atom was acomplex structure made of constituentsthis hypothesishad already been adopted by a number of chemists andThomson himself the previous year to explain the lawof absorption of x-rays but also by the hypothesisthat the cathode rays represented one of theseconstituents (i.e. a component of the atom with negativecharge) that had left the atom. This atomic constituent(which was to become our electron) Thomson calledthe corpuscle. He thus concluded the debate betweenaetherial and material theories by stating:
We have in the cathode rays matter in a new state,a state in which the subdivision of matter is carriedvery much further than in the ordinary gaseous state: astate in which all matter derived from different sourcessuch as hydrogen, oxygen, etc, is of one and the samekind; this matter being the substance from which allthe chemical elements are built up. [ ] If, in thevery intense field in the neighbourhood of the cathode,the molecules of the gas are dissociated and are splitup, not into the ordinary chemical atoms, but intothese primordial atoms, which we shall for brevity callcorpuscles; and if these corpuscles are charged withelectricity and projected from the cathode by the electricfield, they would behave exactly like the cathode rays.They would evidently give a value of m=e which isindependent of the nature of the gas and its pressure,for the carriers are the same whatever the gas may be.
What remained was to measure the charge or the massof these corpuscles separately. As we will see, thistask Thomson performed in 1899; he thus confirmedthe hypothesis of the existence of a negatively chargedatomic component.
3. The charge of the corpuscle
In 1874 Stoney , interpreting the laws of Faraday onelectrolysis in the light of the valence theory proposedby Kekule a few years before, managed to identify theexistence of a defined quantity of electricity by meansof which atoms seemed to combine chemically. Thisdefined quantity of electricity (later called electronby Stoney) was estimated by Stoney to be equal to1:03 1021 emu. Other estimates in the framework ofelectrolysis arrived at values between 4.31 and 4:71 1020 emu .
Wiechert  used the thus defined quantity ofelectricity in January 1897 to interpret the nature ofcathode rays. Although he had only been studying thistopic for a short time, he adopted a material conceptionof these rays, and hypothesized that they consisted ofnegatively charged particles having a charge value equalto 1 electron. To establish if these particles werechemical atoms, or groups of atoms, or some otherbody, Wiechert attempted to estimate their mass, from
deflecting them in a magnetic field. He obtained, inparticular, the following expression (which was lateralso used by Thomson) for a magnetic field B placedperpendicularly to the rays:
m D Bre=v (4)where m; e; v, and r denote, the mass, the charge, thevelocity, and the radius of curvature of the materialcharged objects, respectively. Since e had been fixeda priori, in order to obtain m from (4) it was necessaryto measure v. Wiechert managed to estimate only anupper limit and a lower limit for v, and thus derived arange of values for m. To estimate the upper limit ofvelocity he assumed that all the energy obtained fromthe electric field in the discharge was transformed intokinetic energy. Then he obtained a maximum velocityof 108 m s1 and, with equation (4), a lower limit forthe mass, namely 1/4000 of the mass of the hydrogenatom. The minimum velocity followed from havingthe cathode rays interact with an electromagnetic wave(produced by the same alternating current used to obtainthe cathode rays). Wiechert measured the time it tookthe cathode rays to transit over a distance of 20 cm. Inthis way he concluded that the velocity of the cathoderays was greater than 3 107 m s1; consequently,their mass was less than 1/2000 of the mass of thehydrogen atom. He therefore concluded:
So far as the cathode rays are concerned, they cannotbe atoms as they are known in chemistry, as their massis 20004000 times smaller than that of hydrogen, orlighter than the known chemical atoms.
In judging about Wiecherts conclusion, whichpreceded Thomsons conclusions by several months, itmust be remembered that it rested on the assumptionthat the charge of the cathode rays was identical withthat of the electron of electrolysis. In 1897 such anassumption cannot be considered to be evident. In anycase, Thomson, having obtained the mass/charge ratio ofthe cathode rays and concluded that they were particlesmuch smaller than the hydrogen atom, embarked on aprogram of research aimed at measuring their charge, inorder to establish how much smaller than the hydrogenatom they were.
This program was not an easy one, as Thomson im-mediately encountered serious difficulties in performingthe necessary measurements on cathode rays. Indeed,he had to look for another phenomenon, which he finallyfound in the photoelectric effect: he first gave a newinterpretation to this effect in terms of emission of cor-puscles; second, he obtainedby means of a method Ishall not explain herea value for the mass/charge ra-tio of these corpuscles having the same order of mag-nitude as that for cathode rays; finally, in 1899 hemanaged to devise a system to measure the charge .It should be added that this procedure was based onThomsons experiences since 1890 (as director of theCavendish Laboratory) on the problem of discharge ingases, in which he had involved a number of young re-search associates including C T R Wilson, E Rutherford,J S Townsend and J Zeleny.
138 N Robotti
The methods devised by Thomson were based on thefact that corpuscles, when produced by means of thephotoelectric effect, emitted into a gas saturated withwater vapour and subjected to a process of expansion,behaved like condensation nuclei and each of them couldbe isolated in water droplets. Thomson counted, on theone hand, the total number of droplets observed, whichcorresponded to the number of corpuscles present; onthe other hand, he measured the total charge present.From both results he estimated the charge of a singlecorpuscle.
In this way he finally obtained a value for the chargeof 2:3 1020 emu: hence it assumed the size ofthe unitary charge in electrolysis that had been calledelectron. By inserting this value into the mass/chargeratio previously measured, Thomson found the mass ofthe corpuscle to be 3 1026 g. After many years oflabour on the problem of discharge in gases, he finallyachieved what he called in a letter to Rutherford the: : : direct proof of the existence of masses only 1/1000of the mass of the hydrogen ion .
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 De La Rue W and Muller H W 1880 Experimentalreasearches on the electric discharge with the chlorideof silver battery Phil. Trans. 171 65116
 Faraday M 1838 Experimental researches in electricityPhil. Trans. R. Soc. 128 125
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Whittaker E T 1989 A History of the Theories of Aetherand Electricity 2nd edn (New York: Dover)
Falconer I 1987 Corpuscles, electrons and cathode rays:J J Thomson and the discovery of the electron Br. J.History Sci. 20 24176
Owen G E 1956 The discovery of the electron Ann. Sci.11 17282
Pais A 1986 Inward Bound (New York: OxfordUniversity Press)
Feffer S M 1990 Arthur Schuster, J J Thomson and theDiscovery of the Electron Hist. Studies Phys. Biol.Sci. 20 3361
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